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Title: Development of a microbially induced calcite and silica bio-grout for the sealing of fine aperture fractures
Author: MacLachlan, Erica
ISNI:       0000 0004 6422 6949
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2017
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Geological repositories are being considered as the best feasible solution for the storage of hazardous materials such as high level nuclear waste throughout the world, including the UK. However; when crystalline rock is the chosen storage medium, the construction of the underground tunnels and caverns can enhance discontinuities within the rock. These discontinuities can be pathways by which radio-nuclides can reach the biosphere, due to their higher permeability, connectivity and density (Blyth and Freitas, 1992). Thus, depending on aperture, density and predicted travel times, it may be necessary to grout all fractures, even small aperture ones, which over thousands of years can contribute significantly to subsurface flow. Conventional cementitious and chemical grouts are unsuitable within some regions of a geological disposal facility due to concerns regarding longevity, toxicity, reactions with other barriers and/or workability issues. The four main requirements of a grout are; to be of low viscosity as the lower the viscosity the easier it is to achieve good penetration, to have a controllable gel/setting time, to be chemically inert to prevent reactions within the subsurface or have any toxic consequences during preparation, and to be durable thus able to withstand exposure to varying physic-chemical condition. MICP (Microbially Induced Calcite Precipitation) and Colloidal Silica are novel grouts which may be suitable for the sealing of fine aperture fractures in rock. MICP research has been predominantly focussed on its application in sediments, whilst colloidal silica has shown its potential for reducing the liquefaction potential of non-cohesive soils and for sealing fractures. This research examines the influence of hydraulic controls (velocity, flow rate, aperture) on the spatial distribution of microbially induced calcite precipitation (MICP) within simulated fractures using flocculated Sporosarcina pasteurii. The experimental results show that under flowing conditions, the spatial distribution of microbially induced calcite precipitate on fracture surfaces is controlled by fluid velocity. Even for a uniform initial fracture aperture with a steady flow rate, a feedback mechanism existed between velocity and precipitation that resulted in a precipitate distribution that focussed flow into a small number of self-organizing channels which remained stable. Ultimately, this feedback mechanism controlled the final aperture profile which governed flow within the fracture. To use MICP for field scale sealing operations (e.g., in aquifers and host rock surrounding nuclear waste storage sites), it is important to develop an injection strategy that ensures microbially precipitated calcite is distributed homogenously throughout the rock body to avoid preferential flow through high porosity pathways. Sporosarcina pasteurii was found to be able to hydrolyse urea for several days before the bacteria became encased within calcite preventing access to the cementing fluid. The higher rates of urea hydrolysis occurred within the first 9 hours, though significant rates of urea hydrolysis still occurred after this period. By reducing the size of bacterial flocs it is possible to reduce the impact of sedimentation and straining, promoting a more even distribution of bacteria thus calcite precipitate throughout the plate. By increasing the length of time that the bacteria flow through the fracture, more bacteria can become entrained upon the fracture surface giving a better distribution. The introduction of a filler (colloidal silica) that can also act as a nucleation site for calcite precipitation was examined as a way of reducing the time it takes for the sealing of a fracture. Both Sporosarcina pasteurii and colloidal silica have negative surface charges thus colloidal silica could be used as a nucleation surface, this plus its nanometre size which could allow for a better distribution of and could enhance calcite precipitation. A clear difference in the mass of grout retained within the fracture was seen, with MICP alone showing the greatest weight increase. During the 8 grouting cycles with MICP + colloidal silica there appeared to be pieces of calcite travelling through the open channels. This would indicate that the calcite is unable to attach to the fracture surface. Thus, adding a small amount of colloidal silica to the cementing solution as a filler was not an efficient way to produce calcite fill. However, Sporosarcina pasteurii produces ammonium ions from the hydrolysis of the non-ionic urea, which as a cation can destabilise the silica sol resulting in gelation. Batch tests were used to determine what differences in gel point, gel rate and shear strength were created by different cations, including the chemical addition of ammonium ions and the biological production of ammonium ions by the bacterium Sporosarcina pasteurii. The sensitivity of colloidal silica to calcium chloride can result in dramatic differences in gel time with small changes in molarity having great impact on whether the colloidal silica gels or not. The direct addition of ammonium salts requires ten times the concentration, compared to CaCl2, to achieve similar shear strength values. However; this concentration produces very short gel times, potentially reducing the radius of penetration. The bacterial in-situ production of ammonium ions gives the greatest gel times yet still produces the same shear strength as that of a sodium chloride accelerator. This increasing of gel times, without adversely impacting grout properties, could be beneficial for penetrating greater distances into fractured rock reducing the number of injection points required. This would be particularly useful for subsurface engineering applications where large volumes of rock are required to be grouted.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral